Paper may be strengthened by means of adding strength agents in the paper making process, for example into the furnish at what is known as the wet end of a paper machine or as a coating at the size press unit or coater, which is situated at the dry end of the paper machine. Various types of starches are used in both applications, viz: cationic or amphoteric starches for wet end addition and oxidised or acid modified starches at the size press. Another means of adding starch to strengthen paper is to spray a starch slurry between the plys on multiwire paper machines. Casein and/or latexes have also been used at the coating end of paper machines particularly in conjunction with pigments to improve opacity, whiteness and strength.
Starch is oxidised under acid conditions in order to obtain the necessary starch hydrolysis and viscosity reduction as required in the paper industry.
Starches used in paper coating compositions, whether gelatinised, hydrolyzed, oxidised or otherwise modified, exhibit sporadic unpredictable behaviour which typically results in inconsistent coating properties associated with batch variability. For example, starch prepared from water damaged wheat may give low viscosity coatings when subject to oxidation.
Starch is an extensively chemically purified product obtained from various agro-industrial crops, including maize, potato, wheat and tapioca. For example, maize starch may be prepared from maize by steeping the maize in sodium metabisulphite to soften protein constituents. The steeped maize is then passed through a roller mill to separate protein, wet milled or ground into a starch slurry, screened to remove fibre, and then flash dried.
Starch production involves significant energy use and waste production, and is accordingly a costly processed product.
In an attempt to overcome the problems of gelatinised, hydrolysed and modified starch, Bassie et al (U.S. Pat. No. 6,517,625) describes aqueous dispersions for coating paper stock which comprise 20 to 45% by weight of a filler such as clay, calcium carbonate, talc, ungelled starch or mixtures thereof, and from 16 to 18% wheat gluten. The gluten of Bassie is reduced to cleave some of the disulfide bonds using an acidic reducing agent such as alkali metal sulfites, alkali metal bisulfites, alkali metal metabisulfites, sulfur dioxide, mercaptan and cysteine. Sodium hydroxide is added to a an aqueous solution of the gluten and then the reducing agent is added. The next step is the addition of granular starch, that is ungelatinised starch, to the dispersion. The finished composition has a pH in the range 9–12 such as pH of 9.5–11. The use of a reducing agent such as sodium metabisulphite in Bassi et al raises allergy concerns associated with sodium metabisulphite or other reducing agent residues. The costs involved in providing purified components and subsequent reduction processes according to Bassie et al are not associated with improved processes for the preparation of paper nor does it ensure the paper prepared has improved strength or durability. Furthermore, pHs above 9 in final compositions such as those described by Bassi et al disadvantageously affects paper properties, particularly paper strength.
Wheat flour has also been proposed for use as an additive at the beginning stages of paper production, particularly addition to wet pulp in the uncooked form. Retention of flour in these proposals was often less than 60% w/w, which was uneconomic.
It has also been proposed to use cooked flour at the sizing step of manufacture of paper. However, the insoluble gluten results in fouling of the paper production equipment. A further problem with these proposals is that the gluten content causes a gummy finish in the resultant paper.
U.S. Pat. No. 6,022,450 (Van Kessel et al) describes methods for manufacturing paper or cardboard from a fibre matrix and flour comprising subjecting the flour to a deamidation reaction and/or partial hydrolysis degradation treatment. Flour is mixed with ammonium persulphate under acidic conditions, achieved for example by the addition of acetic acid or citric acid. Flour degradation may also be carried out according to Van Kessel et al by treatment with amylase enzymes, or combinations of amylase enzymes with ammonium persulphate.
In comparative testing with conventional starch coatings, Van Kessel et al found that the performance properties of paper treated according to their methods were either equivalent to those properties conferred by conventional starch coating, or somewhat less effective. For example burst and breaking strength of paper treated according to the methods of Van Kessel et al were respectively 13% and 17% less effective than standard starch coating treatments.
A large number of different types of paper size compositions have been developed in attempts to increase the strength and durability of paper, and to improve printability, opacity and whiteness. For example, U.S. Pat. Nos. 5,122,568 and 5,139,614 teach the use of styrene acrylic copolymers for enhancing sizing and ink print contrast. U.S. Pat. No. 3,562,102 discloses amine reaction products of alkyl glycidyl mixed esters for use in sizing paper substrates. U.S. Pat. No. 4,294,704 describes paper and paperboard coating compositions which contain aqueous latex binders of synthetic polymers, such as styrene-butadiene copolymers, carboxyl styrene-butadiene copolymers and the like in order to provide improved dry and wet tear resistance. U.S. Pat. No. 6,494,990 describes coating compositions based on hydrophilic polyacrylimides and various copolymers. Issues of expense and health concerns of such compositions arise. For example, polyacrylimides have been associated with neurotoxicity.
There remains a need for low cost, high performance, non-noxious paper coating compositions.
This invention has particular applicability to the end stages of paper production, for example the size press coating stage, as well as other stages of paper, paperboard and cardboard production, as well as the production of other products. Coating compositions also find use as adhesives. This invention thus has application in the adhesive field, for example in gummed tape manufacture, and as an additive to gypsum board to improve bonding of the liner to the gypsum core.